Data bases, such as those with large amounts of stored information or which are part of an electronic mail system, often limit access by requiring an alleged authorized user to provide the proper password. The password may be the same for all authorized user's, or may be unique to each authorized user (in which case the authorized user also gives his identity). Fraudulent access to data bases is often the result of an unauthorized person learning the password by monitoring the telephone line between an authorized user and the data base. A system which prevented an unauthorized person, who taps a telephone line or other channel extending between an authorized user and a data base, from using information gained by tapping the line to gain access to the data base, would be of considerable value.